Biosorption may be simply defined as the removal of metal or metalloid species, compounds and particles from solution by biological material. This is a general definition which takes no account the mechanistic details and, although virtually all biological material has a significant biosorptive ability, the concept and application of biosorption has mainly been directed towards microbial systems. Micro-organisms, including bacteria, algae, fungi and yeasts, efficiently accumulate organics, heavy metals and radionuclides from the external environment. The amounts taken up are relatively large and a variety of mechanisms may be involved, including adsorption as well as processes dependent on metabolism. Living and dead cells as well as excreted or derived products, e.g. cell wall constituents, pigments and polysaccharides, are also capable of contaminant removal from solution.
Biosorption is currently of industrial interest because the removal of toxic organic compounds, heavy metals and radionuclides from liquid waste streams can result in detoxification and, therefore, safe environmental discharge. Subsequent treatment of loaded biomass can enable recovery of valuable elements, or further containment of highly toxic and/or radioactive species. With accelerated depletion of natural mineral sources, there is a greater need of recycling of metals while ore efficient means of effluent detoxification must be devised for environmental protection.
Biomass-based technologies provide an alternative or supplement to conventional methods of metal removal or recovery including ion exchange resins, recrystallization, reverse osmosis, precipitation and electrodialysis.
The walls of bacteria, algae and fungi are efficient metal biosorbents and in many bases, initial binding may be followed by inorganic deposition of increased amounts of metal, even up to 50 percent of the dry weight. Ionic and covalent bonding may be involved in biosorption, with constituents such as proteins and polysaccharides also playing important roles. In several species, biosorption may be the largest proportion of total uptake. This is especially true for metals such as lead (Pb) and aluminum (Al) and radionuclides like Uranium and Thorium. Variations in the composition of microbial cell walls, which can be influenced by cultural conditions, can result in considerable variation in biosorptive capacity which enable some degree of selective accumulation, although biosorption may be largely determined by the chemical behavior of the metal species involved. Particulate material, as well as dissolved metals, can also bind to biomass, e.g. suphides of copper, zinc and lead, zinc dust and ferric hydroxide.
For industrial applications, using freely suspended microbial biomass has several disadvantages, including small particle size and low mechanical strength, while a similarity of density with the effluent can limit the choice of bioreactor design even to the point of making biomass/effluent separation difficult. Immobilized biomass has a greater potential in a packed-bed or a fluidized-bed reactor with the benefits including control of particle size, better capability of regeneration and re-use of the biomass, ease of separation of effluent from the biomass for recirculation, high biomass loadings and minimal clogging in a continuous flow system. Furthermore, immobilized systems may be mathematically defined by reference to such parameters as flow rate, metal concentration and loading capacity. Immobilized systems particularly lend themselves to non-destructive recovery, and after metal-loading, the metal may be concentrated in a small volume of solid material or desorbed into a small volume of eluant for recovery, disposal or containment.
The method of immobilization has a significant influence on removal efficiency. Simple electrostatic adsorption of cells to surfaces is weak, affected by pH, and prone to washout by the flow of effluent over the media. Chemical coupling engenders toxic symptoms which reduce efficacy of the biomass. Prior art entrapment of cells within alginates, polyacrylamide and silica gels can be highly efficient in small-scale systems, though diffusional limitations are a problem.
There is, therefore, a need for an improvement over the known art to overcome the current disadvantages and to provide an economical solution.
The invention herein solves these problems and is compatible with liquid systems.
The invention specifically outlines the process of combining a bioagent, for example the bioagent, non-viable Datura innoxia cells and a polyurethane binder, as taught in U.S. Pat. No. 5,120,441. To those skilled in the art, it will become apparent that substitution of Datura innoxia plant cells with other plant species or other biomass (herein collectively referred to as an "active material") may be used in the present invention without detracting from breadth or scope of the invention herein.
Active ingredient biomass materials are well known to those skilled in the art and are further defined in Biochemical Nomenclature, Biochemical Society 1978, Enzyme Nomenclature, 1984 Academic Press, Orlando, Fla.
Use of biomass algae for removal of pollutants from a power plant discharge is disclosed in U.S. Pat. No. 5,011,604. Active ingredient biomass materials immobilized with the present invention include, but are not limited to the bioagents listed below:
______________________________________ Datura innoxia Plant cells Casein Hydrosolate Enzymes Cyanidium Algae Green Tea Plant cells Pseudomonus Bacteria ______________________________________
Many binder systems might look appealing, but the hydrophilic characteristics of the present invention allow the effluent stream to come into intimate contact with the immobilized biomass. This is explained by the fact that any blocking of metal-binding sites of the biomass by the hydrophilic binder is offset by the hydrophilic nature of the described binder allowing the effluent to pass readily through the film of hydrophilic binder to come into contact with the surface of the biomass material suspended in the composite material of the present invention active ingredients.
It is well known that a reduction in particle size, while still maintaining the same weight volume, increases the surface area. However, discrete micron sized individual biomass particles rapidly pack in columns when subjected to increased liquid pressure and such packing causes significant pressure drop. Therefore, larger granules of biomass are used in a conventional filtration device to reduce the pressure drop. Utilizing the invention herein, binds together micron sized discrete individual biomass particles, containing the active ingredients, which allows a reduction of biomass size relative to prior art designs, increasing the available surface area and maximizing the available metal-binding or contaminant adsorption sites.
Envision in a particular prior art format, activated carbon is used as an active absorbent, a 1/8" diameter particle whose outside surface is the only area available to react with the effluent. The biomass sites on its outside surface in contact with effluent removes metal and/or noxious materials from the effluent. However, the interior volume of the activated carbon does not contact the effluent and is useless. Compare this to a powder of smaller particle size of the same weight of activated carbon. The available surface area of the smaller particle sample increase up to 1500 times depending on particle size. The surface to volume ratio of the smaller particle is much higher and up to 90-95% of the available binding sites will be available to interact with the effluent.